Separate eggs. Allow yolks, whites, and 4 Tbsp. of the butter to stand at room temperature for 30 minutes. In a bowl sift together flour, baking powder, and salt. Set aside.
In a 9- or 10-inch cast iron or other oven-going skillet melt the remaining 4 Tbsp. butter. Add the 3/4 cup maple syrup; simmer over medium heat for 5 minutes. Remove from heat. Stir in ginger. Arrange apple rings in the skillet.
In a large bowl combine 1/2 cup of the maple sugar, the egg yolks, and the 4 Tbsp. softened butter. Beat with an electric mixer on low to medium until light and fluffy. Alternately add flour mixture and milk, beating after each addition.
Wash beaters. In another medium bowl beat egg whites with an electric mixer on medium to high until stiff peaks form (tips stand straight); beat in the remaining 1/2 cup maple sugar. Stir about one-third of the egg white mixture into the batter. Fold in the remaining egg white mixture. Spread batter evenly over apples.
Bake in a preheated 350-degree oven for 40 to 45 minutes or until a wooden toothpick inserted in the center comes out clean. Cool in skillet on a wire rack for 10 minutes. Invert onto serving plate. Serve warm with Maple Whipped Cream. If desired, drizzle with additional maple syrup.
Maple Whipped Cream: In a chilled medium mixing bowl combine 2 cups whipping cream and 1/4 cup pure maple syrup. Beat with an electric mixer on medium to high until soft peaks form (tips curl).